From monotypic to multispecies: Integrative data redefine Rheum nobile complex (Polygonaceae) in Sky Island-Alpine habitats

• Overturns the long-held view of Rheum nobile as a single species, revealing it to be a complex of four distinct species. • The four newly identified species correspond to four geographic regions: the Central Himalayas, Eastern Himalayas, Hengduan Mountains, and Gaoligong Mountains. • Reveals conflicting signals between nuclear and plastid genomes, indicating a complex evolutionary history involving gene flow. • Species delimitation is strongly supported by an integrative approach combining morphometrics, multi-locus DNA, and Bayesian coalescent-based analysis (BPP). • Provides a taxonomic revision of the group, including updated morphological descriptions for the four newly recognized species.